Satellite watchers have identified thousands of suspected incidents of potentially illegal transshipping, where fishing ships secretly transfer unauthorized product to refrigerated cargo vessels. These are seen as a major factor in the prevalence of illegal fishing, which is thought to account for at least 20% of seafood worldwide.

Global Fishing Watch, a joint project between Oceana, SkyTruth, and Google, identified 5,065 likely incidents between 2012 and 2016, with the majority occurring in a few hotspots.

Here's a map of the hotspots, which include coastal areas by Russia, Japan, Norway, Argentina, and Peru.

Transshipping hotspots from 2012 to 2016
Oceana

Not all transshipping is illegal, but apparently a lot of it is. Likely transshipping incidents were identified by looking at refrigerated cargo vessels, or reefers, that spent more than three hours within 500 meters of another vessel while traveling at less than two knots and more than 20 nautical miles from shore.

A lot of transshipping occurred just outside national waters, where there's less regulation. Of the transshipping that occurred inside national waters, 82% of it occurred in Russia, parts of which apparently don't have much regulation either.

Below, you can see close-ups of incidents just outside national waters by Argentina, Peru, and northwest Russia (but not northeast Russia):

Oceana

Oceana recommends regulations to limit transshipping and improve vessel tracking and identification, as well as the adoption of global catch documentation schemes.

"Market states need to ensure that the seafood entering their borders is legally caught," Oceana senior campaign director Beth Lowell wrote in a statement. "Some markets already have requirements in places such as the European Union. The United States recently issued new requirements that will begin to address illegal fishing; they will be implemented starting in January 2018."